Chapter 1

98 Everything You Ever Wanted to Know About DVD
to its original width. Alternatively, many new 4:3 TVs can reduce the vertical
scan area to restore the proper aspect ratio without losing resolution (an
automatic trigger signal is sent to European TVs on SCART pin 8). Even
though almost all computers have 4:3 monitors, they have a higher resolution
than TVs so they can display the full widescreen picture in a window
(854�480 pixels or bigger for NTSC, and 1024�576 or bigger for PAL).
Anamorphic video can be converted by the player for display on standard
4:3 TVs in letterbox or pan and scan form. If anamorphic video is
shown unchanged on a standard 4:3 display, people will look tall and skinny
as if they have been on a crash diet. DVD players’ setup options allow viewers
to indicate whether they have a 16:9 or 4:3 TV. In the case of a 4:3 TV, a
second option lets the viewer indicate a preference for how the player will
reformat anamorphic video: automatic letterbox or automatic pan and scan.
For automatic letterbox mode, the player generates black bars at the top
and bottom of the picture (60 lines each for NTSC, 72 for PAL). This leaves
three-quarters of the height remaining, creating a shorter but wider rectangle
(1.78:1). In order to fit this shorter rectangle, the anamorphic picture is
squeezed vertically using a letterbox filter that combines every 4 lines into
3, reducing the vertical resolution from 480 scan lines to 360. (If the video
was already letterboxed to fit the 1.78 aspect, the mattes generated by the
player will extend the mattes in the video.) The vertical squeezing exactly
compensates for the original horizontal squeezing so that the movie is
shown in its full width. Some players have better letterbox filters than others,
using weighted averaging to combine lines (scaling four lines into three
or merging the boundary lines) rather than simply dropping one out of every
four lines. Widescreen video can be letterboxed to 4:3 on expensive studio
equipment before it’s stored on the disc, or it can be stored in anamorphic
form and letterboxed to 4:3 in the player. If you compare the two, the letterbox
mattes will be identical, but the picture quality of the studio version
may be slightly better. (See Chapter 1’s “How Do I Get Rid of the Black Bars
at the Top and Bottom?” for more about letterboxing.)
For automatic pan and scan mode, the anamorphic video is unsqueezed
to 16:9, and the sides are cropped off so that a portion of the image is
shown at its full height on a 4:3 screen by following a center of interest offset
encoded in the video stream according to the preferences of those who
transferred the film to video. The pan and scan “window” is 75 percent of
the full width, which reduces the horizontal pixels from 720 to 540. The pan
and scan window can only travel laterally. This does not duplicate a true pan
and scan process in which the window can also travel up and down and
zoom in and out.
Auto pan and scan has three strikes against it: it doesn’t provide the
same artistic control as studio pan and scan, a loss of detail occurs when
the picture is scaled up, and equipment for recording picture shift informa-